Phase locked loops are well known in the prior art as a means of detecting a desired RF transmitted signal. Such loops detect a signal by comparison of a locally generated estimate of the signal with the received signal itself. The error in phase between the two signals is low-pass filtered (i.e., integrated), and then used to correct the local estimate to resemble the average frequency and phase of the received signal.
In general, in most radio receivers, such phase locked loops are used to lock on to the IF signal, which comprises the originally received carrier signal as altered in frequency through the IF stage, as well understood in the art. At least one prior art reference, however, suggests that phase locking can be made a function of the locally generated estimate and the incoming carrier signal to the IF stage, thereby allowing the phase locked loop to lock with respect to the incoming signal itself.
In the early 1960's, a tangent function phase locked loop was proposed. This loop phase compared an IF stage output signal with a locally generated estimate of the signal in a dual phase comparator that yielded both the sine and cosine of the phase difference between the IF signal and its estimate. These outputs could then be appropriately combined to yield a tangent function (sine divided by cosine), which resultant signal could be used as the correction signal for the loop. Since a tangent function varies its amplitude more significantly than a sine or cosine function, operation of a phase locked loop as a function of the tangent function led to increased phase comparator range and linearity, at least under some operating conditions. An example of a commercially available radio receiver part that makes use of tangent locking principles to lock on to the IF signal is the Motorola MC13020 AM Stereo Decoder integrated circuit.
To date, no one has succeeded in establishing a satisfactory tangent based phase locked loop through the IF stage of a radio receiver. Given the stability, speed, range, and parts improvement that such an arrangement would provide, a need for such a configuration clearly exists.